Controlling or limiting access to many functions is a long-standing practice. For example, mechanically operated locks, both keyed and combination, have been employed to restrict access to doors, and the like, to those that have the proper key or know the proper combination.
A major problem with mechanical locks is encountered when it becomes necessary to alter the mechanism to accept a different key or combination. Such situations arise when keys become lost or it is believed that the key or combination has fallen into unauthorized hands. Often, skilled workmen are required to perform the necessary alteration which may result in an undesirable delay. Also, unavailability of parts may cause even further delay.
Some of the difficulties with mechanical systems have been addressed by electronic lock systems which employ coded elements. Such coded elements have taken the form of cards which are typically magnetically encoded. These electronic systems can be designed to facilitate a change in the access code. Cards, however, are subject to loss, and, like mechanical keys, pilferage. Other electronic systems have employed memory circuits in which preselected access codes are stored. In addition to the memory devices, such systems require additional circuitry for addressing the various memory locations.